1. Field of the Invention
The present invention generally relates to an image forming apparatus such as a copier, a printer, a facsimile machine, and a multifunction machine including at least two of these functions.
2. Discussion of the Background Art
In general, an electrophotographic image forming apparatus such as a copier, a printer, a facsimile machine, etc., includes an image carrier on which an electrostatic latent image is formed, a developing unit to develop the electrostatic latent image with a developer, and a transferer to transfer the developed image onto a recording medium such as transfer sheets. Such an image forming apparatus further includes a surface moving member, such as a sheet transfer belt and an intermediate transfer belt, whose surface moves endlessly.
Known full color image forming apparatuses include a plurality of image carriers on each of which a different color toner image is formed. The respective toner images on the image carriers are then transferred and superimposed one on another directly on the recording medium that is transported by the sheet transfer belt, in what is known as a direct transfer method, or primarily on the intermediate transfer belt and then transferred secondarily onto the recording medium, in what is known as an intermediate transfer method.
Such image forming apparatuses are typically provided with functions to adjust image density, color deviation, and image position. In a known method, a test pattern of toner images is formed on a detection area provided at an end portion of such a surface moving member and detected by an optical sensor, and image density and/or registration of colors are adjusted according to results of the detection.
One known image forming apparatus forms the test pattern in a space between sheets on the surface moving member while images are successively being formed on multiple sheets. However, such as approach requires a relatively long interval between sheets so as to have an area for the test pattern between sheets, which reduce image formation continuous productivity.
In another known image forming apparatus, each of the image carrier, a developing roller, a transfer roller, etc., has an axial length greater than a maximum width of sheets usable therein, and thus an area over which sheets do not pass (non-sheet area) is available on the surface moving member. The image forming apparatus forms test patterns on the non-sheet areas of the surface moving member, and the optical sensor detects the test patterns.
When the non-sheet areas are provided on the surface moving member, the test patterns can be formed parallel to images to be transferred onto the sheets in a main scanning direction, and thus the interval between sheets is relatively short. However, when the non-sheet areas are provided, the lengths of the image carrier, the developing roller, the transfer roller, etc., are increased in the axial direction, and thus the image forming apparatus becomes larger and its cost is increased.